The G1 phase of the cell cycle is the target of extracellular signals that regulate cell proliferation. Mitogenic stimuli are responsible for cell cycle progression via the orderly activation of cyclin-dependent kinases (CDKs) that inactivate members of the retinoblastoma or pocket protein family, pRB, p107 and p130. The understanding of the regulation of the G1 phase of the cell cycle is critical since alterations of its regulation could lead to unscheduled DNA synthesis and tumor formation. The pocket proteins form complexes with the E2F family members to regulate transcription of a specific set of genes, termed E2F-regulated genes, which are required for the passage into S-phase. CDK4,6 and CDK2 are responsible for the inactivation of the repressor activity that pRB/E2F1-3 and p107,p130/E2F4,5 exert on genes required for cell cycle progression. These repressor complexes contain other proteins that contribute to G0/G1 regulation. We recently demonstrated that a novel protein with homologs in all eukaryotes, Mip/LIN-9, participates in the regulation of cell cycle progression. In this application, we will test the hypothesis that Mip/LIN-9, like the pocket proteins, acts downstream of CDK4. We propose that the activity of this G1 kinase is responsible for the release of Mip/LIN-9 from the p107,p130/E2F4 repressor complex and that this step is critical for the association of Mip/LIN-9 with B-Myb is S-phase where both proteins function as transcriptional activators of the G2/M genes. We will also elucidate if Mip/LIN-9 is required for the activity of the repressor complex in G0. Finally, we will produce a new animal model with a complete deletion of the Mip/LIN-9 gene to determine its role in other cellular functions as well as in development. PUBLIC HEALTH RELEVANCE: The goal of this project is to characterize a protein termed Mip/LIN-9, which is responsible for the regulation of cell division and proliferation. This goal is relevant for the growth of all cell types but it is particularly relevant for cancer cells. The goals proposed in this project could produce importance advancement in understanding and treatment of different types of cancer.